I assumed it was an attempt to mask videos from automatic copyright scanners. Like, the software couldn't recognize the video as being copyrighted material if the sound was very different from the original audio.

Youtube itself hosts a library of royalty free music and effects to use in videos. Many creators just use that.

I personally don’t notice the music that much, I’m more annoyed by overused sound effects.

There's libraries of loyalty free music that these things sometimes come out of. Or AI.

Im more concerned about the AI voiceover videos making it impossible to find those really useful low quality videos where some indian guy reviews an obscure piece of tech.

Can't say that I have. Do you have a sample video with the kind of music you're talking about?

I've noticed that clearly CC stuff like HOME gets used quite often... and yt folks often make homages to other users (like Summoning Salts favorite track being used for accomplishments/records now)

Tbf, drain cleaning Australia's synth wave music is pretty rockin but yeah the rest sucks

The weirdest music seems to be middle eastern and Indian/Pakistani videos of things as innocuous as machine shops and auto repair. They blast music over some guy fixing a broken wheel hub.